Michael D. Stenner

The speed of information in a 'fast-light' optical medium.

One consequence of the special theory of relativity is that no signal can cause an effect outside the source light cone, the space-time surface on which light rays emanate from the source. Violation of this principle of relativistic causality leads to paradoxes, such as that of an effect preceding its cause. Recent experiments on optical pulse propagation in so-called ‘fast-light’ media—which are characterized by a wave group velocity υg exceeding the vacuum speed of light c or taking on negative values—have led to renewed debate about the definition of the information velocity υi. One view is that υi = υg (ref. 4), which would violate causality, while another is that υi = c in all situations, which would preserve causality. Here we find that the time to detect information propagating through a fast-light medium is slightly longer than the time required to detect the same information travelling through a vacuum, even though υg in the medium vastly exceeds c. Our observations are therefore consistent with relativistic causality and help to resolve the controversies surrounding superluminal pulse propagation.

Polarization instabilities in a two-photon laser.

We describe the operating characteristics of a new type of quantum oscillator that is based on a two-photon stimulated emission process. This two-photon laser consists of spin-polarized and laser-driven 39K atoms placed in a high-finesse transverse-mode-degenerate optical resonator and produces a beam with a power of approximately 0.2 microW at a wavelength of 770 nm. We observe complex dynamical instabilities of the state of polarization of the two-photon laser, which are made possible by the atomic Zeeman degeneracy. We conjecture that the laser could emit polarization-entangled twin beams if this degeneracy is lifted.

Polarization dynamics of a two-photon laser

The high degree of temporal coherence of laser light arises from a complex interplay between the fundamental light-matter interactions of absorption, spontaneous emission, and stimulated emission. The coherence properties of the generated light can be altered significantly, and often in a surprising manner, by modifying the type of light-matter interaction on which the laser is based. As an example, the two-photon laser [1,2] is based on the higher-order two-photon stimulated emission process, whereby two incident photons stimulated emission process, wherby two incident photons stimulated atom to a lower energy state and four photons are scatted, as shown schematically in Fig. 1. While replacing the standard one-photon stimulated emission process by a high-order one might be expected to give rise to subtle differences observable only at the quantum level, it has been predicted that there will be dramatic changes in both the microsopic laser behavior even when many atoms participate in the lasing process . The reason for these differences is that the two-photon stimulated emission rate depends quadratically on the incident phton flux, resulting in an inherently nonlinear light-matter interaction.

Experimental realization of a two-photon laser in strongly driven potassium atoms

Summary form only given. We present experimental evidence for novel quantum oscillators consisting of strongly driven potassium atoms contained in a high finesse optical resonator. These oscillators are based on two-photon stimulated emission, making their behavior and properties significantly different from conventional lasers, but making them harder to realize. The multi-level structure of alkali atoms is ideally suited for realizing such an oscillator since it offers a wealth of scattering processes that display strong resonant enhancement. Two-photon amplification arises from laser-driven resonant scattering between magnetic hyperfine levels. A novel configuration using different states of polarization and an orthogonal beam geometry allows discrimination between the numerous gain mechanisms.

Observation of large "fast light" pulse advancement without distortion

We observe pulses advanced by 15% of their width and experiencing only minor distortion using laser-driven potassium atoms in a novel configuration that avoids competing nonlinear optical effects.

Pulse propagation in a high-gain bichromatically-driven Raman amplifier

Using intense electromagnetic fields, it is now possible to tailor the absorption. amplification, and dispersion properties of multi-level atoms [1]. Dramatic examples include the reduction of the group velocity of a pulse of light to 17 m/s [2] and increasing the group velocity vg to values greater than c or even to negative values [3]. Very recently, pulses of light have been effectively stopped in an atomic medium and the information initially contained in the pulse is encoded reversibly in the atomic coherence [4].

Dynamics of a two-photon laser

The high degree of temporal coherence of laser light arises from a complex interplay between the fundamental light-matter interactions of absorption, spontaneous emission, and stimulated emission. The coherence properties of the generated light can be altered significantly, and often in a surprising manner, by modifying the type of light-matter interaction on which the laser is based. As an example, the two-photon laser [1, 2] is based on the higher-order two-photon stimulated emission process, whereby two incident photons stimulate an excited atom to a lower energy state and four photons are scattered.

Quantum limits to superluminal advancement

Summary form only given. Using intense electromagnetic fields, it is now possible to tailor the absorption, amplification, and dispersion properties of multi-level atoms. Dramatic examples include the reduction of the group velocity of a pulse of light to 17 m/s, and increasing the group velocity to values greater than c or even to negative values. The interpretation of a negative group velocity is that the peak of the pulse leaves the medium before it enters. We will discuss possible quantum limits to the advancement of the peak of a pulse propagating through a high-anomalous-dispersion medium and our experiments on pulses propagating through a dispersion-tailored potassium vapor.